Catalogue
Laws, heuristics and biases
38 concepts that design calls “laws”, heuristics and biases. Each one carries what to do, why it works and where the same idea turns into manipulation. And each one declares how much was actually measured, because that is where the word starts to look too big.
What the mark next to each law means
- measured, and the study is in the bibliography
- my own deduction from the mechanism; nobody measured it
- I found no variation with the body, and nobody tested for it on purpose
- says nothing about people — it describes the work, not who uses it
A law, in physics, holds everywhere we have looked. These do not. The mark says how much is known about where each one stops holding.
Attention and response time
Why someone takes so long to find the button, freezes in front of a long menu, and gives up when the screen is slow to answer. This is the axis of time and effort: what the interface charges before anybody decides anything.
V1The first area of the brain, right at the back of the head, to receive what the eye sent. It is where the image becomes contrast, edge and orientation, before you recognise what you are looking at.See it in Senses and attention →See in the glossary → filters what arrives from the eyes, the prefrontal cortexThe region behind the forehead, where the person decides, plans and sustains attention. It keeps one thing in focus at a time and inhibits the rest: it is where the bottleneck of conscious decision usually tightens.See it in Memory →See in the glossary → sustains a single focus at a time and the frontoparietal circuitsThe pair that supports working memory: the prefrontal cortex holds the information active while the parietal lobe keeps track of where things are.See it in Memory →See in the glossary → hold what is in use right now. Together, the three set the ceiling on how much a screen can ask.
6 concepts · 6 with an exercise · 5 with a case
- Fitts’s law The time to reach a target is a logarithmic function of the ratio between the distance to the target and the size of the target.
- Hick–Hyman law The time it takes to make a decision increases with the number and complexity of the choices available.
- Doherty threshold Productivity increases when a computer and its users interact at a pace of under 400 milliseconds.
- Cognitive load theory Cognitive load refers to the amount of mental resources required of an individual to process information.
- Miller’s law The magical number seven, plus or minus two: some limits on our capacity for processing information.
- Parkinson’s law Work expands so as to fill the time available for its completion.
Visual perception and organization
Why two elements sitting close together read as one group with nothing saying so, and why one extra highlight makes all the others disappear. Grouping, hierarchy and contrast, settled before any conscious decision.
V1 detects edges and contrasts, V2The area right after V1. It joins the edges V1 detected, separates figure from ground and decides which elements form a group, all of it before you notice you looked.See it in Senses and attention →See in the glossary → decides what is figure and what is ground, and the superior colliculusA station in the brainstem that pulls the gaze towards what moves or contrasts, before the person decides to look. It is that reflex that every visual highlight hijacks.See in the glossary → pulls the gaze towards whatever moves or pops out. Grouping happens before the person realises they have looked.
9 concepts · 9 with an exercise · 9 with a case
- Law of proximity Objects that are close to one another tend to be perceived as belonging to the same group.
- Law of similarity The human eye tends to perceive elements similar in colour, shape or texture as part of the same group, even if they are separated.
- Law of common region Elements tend to be perceived as grouped if they share a visibly bounded area.
- Law of uniform connectedness People tend to perceive elements connected by lines or continuous shapes as part of the same group.
- Law of Prägnanz Perceptual organisation will always be as “good” as the prevailing conditions allow.
- Salience bias People tend to focus on items or information that are more prominent and ignore those that stand out less.
- Clustering illusion People tend to perceive meaningful patterns in random sequences of data.
- Law of legibility Text characters must have sufficient size, style and contrast so that they can be read with minimal effort.
- Law of visibility Necessary objects must be visible without effort so that users can make decisions immediately.
Emotion and aesthetics
Why a beautiful screen feels like it works better than it does, and why a single badly worded error erases ten things you got right. The axis of how the person feels while using it — which is what decides whether they come back.
The amygdalaTwo small structures in the temporal lobe that decide, in milliseconds, whether something is a threat or an opportunity. They respond to your screen before any conscious judgement about it.See it in Emotion →See in the glossary → reacts in milliseconds, the nucleus accumbensThe centre of the reward circuit. When something goes right — a button that responds, a task that closes — it is what records “worth it, do it again”.See it in Emotion →See in the glossary → records what was worth it and the orbitofrontal cortexIt sits right behind the eyes and assigns value: whether the thing is worth it, whether it is pleasant, how much it costs in effort. It answers before the person knows they asked.See it in Emotion →See in the glossary → assigns value. dopamineThe chemical signal of reward and anticipation. It is not the molecule of pleasure, but of “worth seeking”, which is why expectation usually pulls more behaviour than delivery does.See it in Emotion →See in the glossary → and cortisolThe stress hormone, and a slow hormone: it takes twenty to forty minutes to peak, so it is not what explains the irritation with a frozen screen, but rather what builds up over a whole day of bad work. Sustained, it brings down working memory.See it in Emotion →See in the glossary → are the two chemical signals that run through the entire axis.
5 concepts · 5 with an exercise · 5 with a case
- Aesthetic–usability effect Users often perceive aesthetically pleasing design as more usable design.
- Flow state The mental state in which a person performing some activity is fully immersed in a feeling of energised focus, full involvement and enjoyment in the process of the activity.
- Zeigarnik effect Uncompleted or interrupted tasks are remembered more easily than completed tasks.
- Negativity bias Negative events have a greater psychological impact than positive events of the same intensity.
- Temporal discounting Future rewards are perceived as less valuable than immediate rewards, even when they are objectively larger.
Memory and decision
Why nobody remembers what was in the middle of the list, why the crossed-out price next to it changes what counts as expensive, and why changing a pattern irritates even when the new one is better. It is what the person holds on to, and what they decide with what they held.
The hippocampusThe structure that turns recent experience into lasting memory. Everything your interface asks the person to remember tomorrow passes through it today.See it in Memory →See in the glossary → turns what happened into long-term memoryWhat the person still knows tomorrow of what they saw today. It does not live in one place only: it forms through the hippocampus and, over time, comes to live spread across the cortex.See it in Memory →See in the glossary →, working memoryThe mental space where the person holds what they are using right now. It holds about four things, and they fade in seconds if they are not used.See it in Memory →See in the glossary → holds what is in use, the dorsolateral prefrontal cortexThe part of the prefrontal cortex linked to self-control and to decision under uncertainty. It is the one that loses to impulse when the immediate reward is visible on the screen.See in the glossary → chooses under uncertainty and the ventromedial prefrontal cortexIt makes the global judgement, the final opinion on an entire experience, not on each moment of it. It is because of it that the person sums up a ten-screen flow in a single sentence.See it in Emotion →See in the glossary → makes the final judgement on the whole experience. All four were measured in memory and decision tasks, not in the laws of this axis.
9 concepts · 8 with an exercise · 9 with a case
- Jakob’s law Users spend most of their time on other sites. This means that users prefer your site to work the same way as all the other sites they already know.
- Scarcity bias When something is perceived as limited, its subjective value increases, even if its usefulness remains the same.
- Availability heuristic People judge how frequent an event is by how easily they can recall examples of it.
- Peak–end rule The evaluation of an experience does not depend on the average of all its moments, but on the intensity of its most striking point and of its ending.
- Von Restorff effect When multiple similar objects are present, the one that differs from the rest is most likely to be remembered.
- Serial position effect Users are more likely to remember the first and last items in a list than the items in the middle.
- Choice-supportive bias After making a decision, people tend to remember the positive aspects of the chosen option and to forget or play down the negative ones.
- Endowment effect People tend to assign more value to things they already own, even if their ownership is recent or irrelevant.
- Anchoring bias The first piece of information offered on a topic, especially when it involves numbers, works as a fixed reference point, which pulls the estimates made afterwards.
Social behavior and expectation
Why a stranger’s review outweighs your product description, why we blame the user instead of the flow, and why familiarity reads as quality. It is what the person does because other people are around.
mirror neuronsCells that fire both when you do something and when you see someone else doing it. They were proposed as the basis of imitation and empathy, a reading the field itself has already corrected, and what is left is that seeing someone act partly activates the system of the one who acts.See it in The social mind →See in the glossary → read other people’s actions, the mentalising networkThe circuit that infers what goes on in other people’s heads: intentions, beliefs, desires. It is what comes into play when the interface has people on the other side, in the rating, in the comment, in the count of those who have already bought.See it in The social mind →See in the glossary → infers the intention behind them and the medial prefrontal cortexThe part of the prefrontal cortex that thinks about people: what others want, what they think of us, whether they can be trusted.See it in The social mind →See in the glossary → decides whom to trust. It is a circuit calibrated for small groups.
5 concepts · 3 with an exercise · 5 with a case
- Social proof People’s tendency is to regard the behaviour of others as the most appropriate way to act, especially in ambiguous situations.
- Mere-exposure effect Repeating a stimulus makes the brain process it with less effort and, with that, find it more attractive.
- Illusion of transparency People tend to overestimate how much their emotions, intentions and thoughts are perceived by others.
- Fundamental attribution error When explaining other people’s behaviour, people tend to overestimate internal factors, such as personality, and underestimate situational ones.
- Illusion of control People tend to overestimate their degree of control over events that are, for the most part, random.
About the work, not about who uses it
Why complexity never disappears and only changes sides, why what is not needed stays out, why use concentrates in a few functions, and why newcomers and experts need different routes. These four do not describe a person. They describe the work, which is why the question of who they were measured on does not apply to them.
Almost nothing here was measured in a brain, and the four do not need that to hold. The only one that cites a structure, flexibility and efficiency of use, cites what is known about automating a sequence, and declares the bridge.
4 concepts · 4 with an exercise · 4 with a case
- Tesler’s law Every application has an inherent amount of irreducible complexity. The only question is: who will have to deal with it — the user, whoever builds the application or whoever builds the platform?
- Occam’s razor The best solution is the one that presents the fewest possible assumptions.
- Pareto principle In many events, roughly 80% of the outcomes come from 20% of the causes.
- Flexibility and efficiency of use Shortcuts that are invisible to beginners speed up those who have already mastered the system, and make it possible to serve both without choosing between them.